[slate] Fix editor split commands (#35061)
[slate] Fix editor split commands (#35061)

Make depth argument optional in editor.splitBlock & editor.splitInline, as well
as in range commands.

As per https://docs.slatejs.org/slate-core/commands#splitblock
dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped